The optional second argument

You can use the optional second argument of the INIT function as a setup argument to customize the aggregate computation. For example, you could prepare an aggregate that would exclude the N largest and N smallest values from its calculation of an average. In that case, the value of N would be the second argument of the aggregate expression.

The setup expression must come from the group-by columns because the value of the setup should remain the same throughout the computation of the aggregate.

For C, the setup expression cannot be a lone host variable reference.